High-pressure phase behavior of the binary systems

Poot, W. Kruger, K.-M. de Loos, T.W. High-pressure phase behavior of the binary systems (butane + adamantane) and (butane + diaman-tane). J. Chem. Thermodyn. 2003, 35, 583-596. 

In this section the experimental data and modeling of the solubilities of diamondoids in supercritical solvents such as carbon dioxide, methane, and ethane are presented first, followed by the solubilities of these components in liquid organic solvents. In the last part of this section high-pressure phase behavior of the binary systems of diamondoids containing butane and isobutene is explained. 

High-Pressure Phase Behavior of the Binary Systems... 

Foot et al. [22] have determined experimentally the high-pressure phase behavior of the binary systems (butane adamantane) and (butane -I- diamantane). The phase behavior of these binary systems is shown schematically in Figure 1.7. Because the phase diagrams of pure adamantane and diamantane show a solid-solid (si + S2) transition line the curve representing the (solid diamondoids -I- liquid + vapor) equilibrium will split into two branches. One branch corresponds to the (si -f 1 -f v) equilibrium and the other branch corresponds to the (. 2 1 ) equilibrium. Both branches intersect at the (si S2) equilibrium line of the pure diamondoids. The... 

In addition to the above systems, Miltenburg et al. [23] determined the high-pressure phase behavior of the binary system isobutene -I- diamantane according to a synthetic method in the temperature range (320-530) K and the pressure range (0.3-10) Mpa which are presented in Table 1.15 for their experimentally determined LV equilibrium points. 

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

The phase behavior of the binary system ethylene-PEP and the ternary system ethylene-PEP-C02 have been determined in an optical high-pressure cell designed for pressures up to 400 MPa and temperatures up to 450 K. Fig. 8.1 gives a schematic view of the method used for cloud-point measurements. 

Shariati, A. and Peters, C. J., High-pressure phase behavior of systems with ionic liquids Measurements and modeling of the binary system fluoroform + l-ethyl-3-methylimidazolium hexafluorophosphate, /. Supercrit. Fluids, 25, 109, 2003. 

The extraction of the product from the ILs with SC-CO2 is the most important advantage of the biphasic systems. Typically, the effectiveness of SC-CO2 for extraction depends on the phase behavior of the binary IL-SC-CO2 system. The solubility of CO2 in the IL is important to allow for contact between the CO2 and the products. The dissolved CO2 also decreases the viscosity of the IL and therefore improves mass transfer. It has been reported that CO2 is soluble in every IL, whereas ILs are not soluble in the gaseous CO2 phase, even at high pressures (Blanchard et al., 2001). Such systems have been successfully used in the synthesis of esters. 

There are three essential elements that make up the thermodynamic foundations of supercritical fluids. These elements are experimental and identification techniques for elucidating the phase behavior, models for dense gases, and computation methods. 20.1.2.1.1 Experimental methods There are two basic approaches to experimental determination of the high-pressure phase behavior of a system, synthetic and analytic. In the synthetic approach, phase boundaries of a fixed (known) eomposition system are observed, usually visually, in a cell with sight windows, by manipulation of the system pressure and temperature. These experimental systems, one shown as Figure 20.1.10 allow determination of cloud point, dew point, bubble point, and eritieal point of partieular binary systems including polymers. Unlike earlier custom-... 

In addition to the experimental data, the partitioning behavior of MMA between water and CO2 has been modeled. The Peng-Robinson equation of state combined with various mixing rules as described in Section 14.4.1 has been assessed on the ability to correlate phase equilibrium data from literature of the binary subsystems CO2-H2O, MMA-CO2 and MMA-H2O. Subsequently, the model has been used to predict the phase equilibrium behavior of the ternary system CO2-H2O-MMA. Partition coefficients were calculated at four different temperatures at pressures ranging from 5 to 10 MPa. In order to provide a means for comparison, the experimentally determined partition coefficients obtained in the high-pressure extraction unit were used to evaluate the results of the predictive model for phase equilibrium behavior. 

Fluid-fluid phase separations have been observed in many binary mixtures at high pressures, including a large number of systems in which helium is one of the components (Rowlinson and Swinton, 1982). Fluid-fluid phase separation may actually be the rule rather than the exception in mixtures of unlike molecules at high pressures. Fig. 6.4 shows the three-dimensional phase behavior of a binary mixture in schematic form. This diagram includes the vapor pressure curves and liquid-vapor critical points of the less volatile component (1) and the more volatile component (2) in their respective constant-x planes. The critical lines are interrupted one branch remains open up to very high temperatures and pressures. Systems that can be represented by a diagram such as Fig. 6.4, those for which the critical lines always have positive slope in the p — T projection, have been called fluid-fluid mixtures of the first kind. A second class of system, in which the critical line first drops to temperatures below T (l) and then increases, exhibit fluid-fluid equilibrium of the second kind. There is, however, no fundamental distinction between these two classes of fluid mixtures. 

As an example for high-pressure system properties. Figure 2.2 demonstrates the phase behavior of CO2 and Figure 2.3 illustrates the different phases of the binary system C 02-water [9]. 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

Liquid Solution Behavior. The component activity coefficients in the liquid phase can be addressed separately from those in the solid solution by direct experimental determination or by analysis of the binary limits, since y p = 1. Because of the large amount of experimental effort required to study a ternary composition field and the high vapor pressures encountered in the arsenide and phosphide melts, a direct experimental determination of ternary activity coefficients has been reported only for the Ga-In-Sb system (26). Typically, the available binary liquidus data have been used to fix the adjustable parameters in a solution model with 0,p determined by Equation 7. The solution model expression for the activity coefficient has been used not only to represent the component activities along the liquidus curve, but also the stoichiometric liquid activities needed in Equation 7. The ternary melt solution behavior is then obtained by extending the binary models to describe a ternary mixture without additional adjustable parameters. In general, interactions between atoms in different groups exhibit negative deviations from ideal behavior... 

Phase Behavior of Polymer Systems in High Pressure Fluids The basic description and definitions of the different phenomena associated with phase equilibria in polymer solutions are described in Section 25.2.5. Topics such as construction and interpretation of binary... 

Let us now consider solid-liquid equilibria in a binary system. Contrary to phase equilibria between vapor and condensed phases, in the systems comprising solid and liquid phases the pressure as variable can usually be neglected. The behavior of condensed systems does not mostly depend on pressure and thus the appropriate phase diagrams are valid for arbitrary (but not too high) pressures. Thus, phase changes in a binary system can be represented on a temperature-composition diagram as shown in Figure 3.2 for two arbitrary components A and B. 

CR Yonker, JC Linehan, JL Fulton. The use of high pressure NMR for the determination of phase behavior for selected binary solvent systems. J Supercrit Fluids 14 9-16, 1998. 

This work demonstrates that binary data alone do not necessarily indicate some important aspects of the phase behavior, such as melting point depressions of multicomponent systems involving high pressure gases and supercritical fluids. 

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